There has been a demand to enable quilting with a sewing machine, in which stitches are formed while forming decorative patterns on the front side of a three-layered fabric having cotton, feather or urethane foam, or the like, stuffed between a front cloth and a back cloth. In such case, the user may enjoy quilting by combining straight stitches and curved stitches by vertically driving a needle bar and freely moving the three-layered quilting fabric in given directions.
For example, JP 2002-292175 A (pages 3 to 4, FIGS. 3 to 5) describes a sewing machine provided with a downwardly oriented image sensor mounted in a head of a sewing machine arm. The images captured by the image sensor are inputted to a microcomputer during the sewing operation and the sewing machine takes in a part of such images at predetermined small time intervals as static images. Then, a movement distance of the workpiece cloth is calculated by a first interrupt process. Thereafter, a second interrupt process obtains needle-bar movement speed, in other words, rotational speed of a sewing machine motor based on a preset “pitch-width” and the calculated movement distance, and changes the rotational speed of the sewing machine motor to the obtained rotational speed so that stitches formed by manual cloth feed is arranged at an consistent “pitch-width”.
The sewing machine described in the above mentioned JP 2002-292175 A obtains the movement distance of the workpiece cloth during a sewing operation by processing a plurality of static images outputted at small time intervals from the image sensor provided in the head of the arm. However, not only is the image information provided by the image sensor affected by the color and material of the workpiece cloth and the color and brightness of external light radiated during the sewing operation, but may also be deteriorated by the distance between the image sensor and the workpiece cloth. Thus, it is not possible to obtain the movement distance of the workpiece cloth with consistent accuracy, leading to increased difficulty in providing a consistent “pitch-width” of the stitches.
In the light of the above, the image sensor may be placed in close proximity of the workpiece cloth, in other words, near a needle drop position to improve the accuracy in detecting the movement distance of the workpiece cloth. However, placing the image sensor in close proximity of the workpiece cloth may become an impediment to the user during the sewing work, consequently leading to problems such as poor workability.